Reconciling the magnetic field structures seen in variable active galactic nuclei with the unified scheme

Abstract

In the orientation-based unification scheme for active galaxies, the most variable active galactic nuclei-the BL Lacertae objects and highly polarized quasars (HPQs)-correspond to radio galaxies with their relativistic jets oriented close to our line of sight. This otherwise successful scheme has recently been chal-lenged by radio polarimetric very-long-baseline interferometry, which reveals different magnetic field structures in the knots of synchrotron emission identified with shocks in these jets. For BL Lacs the field is generally perpendicular to the jet, whereas no such trend is apparent for HPQs. This dichotomy has led to the claim that luminous BL Lacs and HPQs are inherently different types of object. Here we propose that this difference can be readily explained within the unified scheme by the effect of relativistic aberration: at small viewing angles, the observed knot emission arises primarily from the freshly excited plasma of the shock front, whereas at larger angles, Doppler-boosted emission from the slower, post-shock plasma is the dominant contribution. We identify the former case with BL Lacs and the latter with HPQs.